Linq多表查询,查询结果又有一个集合

上面代码并没有严格写,但是意思是那样的
假设已经有数据集
userSet picSet picTagSet tagSet

---

from t1 in picset
join t2 in userset on t1.userid equals t2.userid
select new {
    picid = t1.picid,
    username = t2.username,
    tagnames = (from t3 in tagpicset 
                join t4 in tagset on t3.tagid equals t4.tagid 
                where t3.picid == t1.picid 
                select t4.tagname),
}